Monolithic investment shell casting



Sept. 21, 1965 J. HocKlN ETAL,

MONOLITHIC INVESTMENT SHELL CASTING Filed July 22, 1963 6 f f ffii,

United States Patent O 3,206,810 MONLITHIC INVESTMENT SHELL CASTHJG John Hockin and Richard l. Charlton, Brooklield, Ill.,

assignors to Consolidated Foundries & Mfg. lCorporation, Chicago, lll., a corporation of Delaware Filed July 22, 1963, Ser. No. 296,511 14 Claims. (Cl. 22-129) This invention relates to monolithic investment shell casting, and more specifically, to a new process for preparing a shell for the casting of non-ferrous metals such as aluminum and copper base alloys, and to an improved monolithic shell formed by such process.

In conventional monolithic shell casting, a pattern formed of combustible or otherwise heat-removable material, such as wax, plastic, frozen mercury or the like, is successively dipped into a slurry of ceramic material to form a hard multiple-layered monolithic mold or shell. The pattern may be removed from the mold by heat and molten metal is then poured into the one-piece mold to form the casting. Thereafter, the casting is removed by dissolving the residual inexpensive mold in caustic soda or some other suitable chemical or by destroying it mechanically or by a combination of both methods.

For reasons of economy in both time and materials, and because of their generally higher quality and smaller size and weight, ceramic shell molds are often more de sirable than the relatively bulky and slow-drying onepiece poured molds (flasks) in which plaster of Paris is usually used as the base investment. However, in the past the use of shell molds has been considered to have substantial limitations. For instance, despite the important advantages of shell casting over flask casting, it has been considered impractical if not impossible to use shell casting techniques in the investment casting of aluminum and other non-ferrous metals.

The principal problem involved in casting aluminum in a ceramic shell is the difficulty in removing the shell after the aluminum ha-s solidilied. Caustic soda cannot be used to remove the shell because of the chemical action on the aluminum casting. Removal by chemical destruction of the shell without also destroying or damagi-ng the aluminum casting is extremely ditlicult and costly and, where cored holes, undercuts and the like are present in the casting, may be virtually impossible.

One of the principal objects of the present invention is to provide a method for forming an investment casting shell particularly suitable for use in the investment casting of aluminum and other non-ferrous metals. More specifically, it is an object of this invention to provide a relatively low cost method for utilizing plaster of Paris in making investment casting shells, thereby combining the advantages of shell casting techniques with the desirable properties of plaster of Paris as a mold-forming material, A still further object is to provide a fast and relatively inexpensive method of forming a multi-layered shell in which the plaster content of all layers is converted to its hydrate-d form after all the layers are in place.

Another object is to provide an investment casting shell in which plaster of Paris is used as a principal ingredient, the shell thereby being sufficiently frangible for mechanical destruction when used in the casting of aluminum and other non-ferrous metals having similar properties. In this connection, it is a specific object to provide a multi-layered investment casting shell which has relatively soft and frangible intermediate layers but which has relatively hard and smooth inner and outer facing layers, the shell as a whole therefore being readily destructible but at the same time having the important advantages of conventional ceramic shells.

Other objects will appear from the specification and drawings in which:

FIGURE 1 is an enlarged broken diagrammatic sectional view illustrating a shell formed by the method of the present invention;

FIGURE 2 is an enlarged diagrammatic sectional view similar to FIGURE 1 but illu-strating a modified shell embodying the invention.'

An important aspect of the present invention lies in the discovery that plaster of Paris may be used in making investment casting shells, and that the method of making such shells may be carried out inexpensively and rapidly, if hydration of the plaster is delayed until all of the layers of the shell are in place. By such a method, a stable slurry containing plaster of Paris may be prepared and maintained for use in forming each of the dipcoats, the plaster in the slurry remaining in dehydrated condition so that the slurry itself will not set or harden in its container.

Another aspect of the invention lies in the recognition that adequate strength in a multi-layered shell prior to hydration of the plaster content thereof may be achieved by mixing the plaster with relatively small amounts of a hydrolyzed ethyl silicate binder, the amount of the binder being carefully controlled to hold the layers of the shell together until the plaster has been converted to its hydrated form but being in suiciently small quantities so as not to prevent subsequent destruction of the shell by conventional mechanical techniques.

Since the ethyl silicate binder serves only to hold the layers together until all of the layers or coats have been applied, and since the finished shell derives its strength principally from the hardened plaster thereof, such a shell is particularly suitable for use in the investment casting of aluminum and other non-ferrous metals wherein chemical shell-removal methods are unsuitable and wherein the shell must be suciently frangible to permit the use of mechanical methods of shell destruction without at the same time damaging the casting.

In practicing the method of the present invention, a stable slurry containing plaster of Paris as a principal ingredient is rst prepared. This slurry essentially comprises plaster of Paris, hydrolyzed ethyl silicate, and a finely-divided refractory material, all in a volatile, nonaqueous carrier. For the carrier, any organic solvent miscible with ethyl silicate might be used; however, it has been found that the lower boiling point alcohols, such as ethyl, methyl, and isopropyl alcohols, are particularly suitable.

Finely-divided fused silica constitutes an effective refractory, although other thermally-stable materials may be used such as, for example, alumina, zirconia, magnesite, silicon carbide, spinel, forsterite, and the like.

The hydrolyzed ethyl silicate may be prepared as a concentrate 4by reacting ethyl `silicate with ethyl alcohol in the presence of controlled amounts of water and concentrated hydrochloric acid. The resulting silicic acid solution is stable and may be stored until use in the 3 preparation of the slurry, at which time the concentrate is diluted with alcohol and is mixed with the plaster and refractory to form a slurry of the proper viscosity.

As an example, the following slurry composition has The liquid portion of the slurry, consisting of the hydrolyzed ethyl silicate concentrate diluted with ethyl alcohol, contains approximately 6.5 percent si-lica by weight. While such a percentage has been found to give excellent results, producing a binder of adequate strength to hold the plaster and refractory together prior to hydration but sufficiently weak to -permit destruction of the final shell, the proportion of silica in the liquid portion of the dipcoat may fall anywhere within the general range of approximately 4 to 15 percent. The preferred range is approximately 5 to 10 percent.

After the slurry has been prepared, a pattern formed from wax, plastic, or any other suitable material is dipped into the slurry. Upon removal, the dipcoat is stuccoed with a refractory grain 'such as 60 mesh alumina. Any other finely-divided refractory such as zirconia, silicon carbide, or magnesite might also be used to provide a rough surface for application of the second dipcoat.

After the iirst dipcoat has at least partially hardened, the pattern is again immersed in the slurry and the stuccoing and drying steps are repeated until a `series of coats or layers have been applied. Referring to FIGURE 1, the `successively applied coats are represented by numerals -15, such coats forming a composite shell which conforms with the surface contour of pattern 16. While the total number of dipcoats may be varied, at least five or six coats are normally required to build up a shell of sutiicient thickness.

Several hours drying time are required for each dipcoat if substantially complete evaporation of the alcohol carrier is desired. It has been -found that the entire process may be expedited if a small amount of a neutralizing -agent is admixed with the refractory grain stuccoed upon each freshly-applied dipcoat. Any weakly basic salt may be used as the neutralizing agent; however, it has been found that ammonium carbonate is particularly suitable because it leaves no alkali residue. The addition of ammonium carbonate or other neutralizing agent raises the pH of the freshly-applied liquid coating and causes the hydrolyzed ethyl silicate content thereof to gel. Gelation of the freshly-applied dipcoat allows a subsequent dipcoat to be applied directly thereon without rst completely removing the residual alcohol from the first coat. Therefore, the setting time between successive coats is reduced by such gelation to approximately one half hour or less.

The amount of ammonium carbonate or other neutralizing additive is yonly enough to neutralize and thereby render unstable the hydrolyzed ethyl silicate solution. For the example given, approximately 50 grams of ammonium carbonate are admixed with each pounds of refractory grain.

After the desired number of dipcoats have been applied, the shell is immersed in water at room temperature for approximately 10 to 30 minutes, thereby hydrating the plaster of Paris and causing it to set. The shell is then allowed to dry.

The shell structure illustrated in FIGURE 2 is identical to the structure already described except that the inner and outer coats 10 and 15 are of different composition than intermediate coats 1214, the latter being -of the same composition as in the first embodiment. In order to form a shell which clings to and conforms closely with the surface of a pattern and which is at the same time readily destructible, it has been found that the use of -plaster may be limited to the intermediate coats and that the compositions of the inner (and outer) coat may be altered to increase controllability of the material. A particularly effective initial dipcoat for the inner facing layer of the shell may be prepared by using a colloidal aqueous silica dispersion as the primary bonding agent in admixture with any suitable ceramic filler in a finely-divided state. Thus, a slurry for the facing coat may have the following composition:

Colloidal silica mls 2,700 Water do 4,050 Wettng agent do 40 Chlorothene do 50 Clay lbs-- 2.5 Fused silica (325 mesh) do 17.5 Tabular alumina (325 mesh) do 30 Chlorothene is used as an antifoaming agent and the clay serves as a suspension agent to prevent the fused silica and tabular alumina from settling. In composition, this slurry is similar to the dips used in forming conventional ceramic shells except that it contains a greatly reduced amount of colloidal silica. Specifically, the amount of colloidal silica is between one half to one third of the amount normally used. To produce a formative facing layer which will conform precisely to the shape of the pattern and which at the same time may be broken away by the application of mechanical forces, it has been found that the silica content in the liquid portion of the first dipcoat should fa-ll within the range of approximately 5 to 20 percent silica.

The final dipcoat I5 is applied after the intermediate layers have been hydrated and the unfinished shell has dried. The composition of the nal coat is the same as the facing coat to the extent that it lacks plaster of Paris as an ingredient thereof and instead depends upon colloidal silica as the primary bonding agent. If desired, the proportion of colloidal silica in the final dipcoat may be substantially greater than in the initial coat and, in general, may be of the same composition as a conventional ceramic dipcoat.

The pattern material may be removed by conventional techniques such as flash firing at 1900 degrees F. Care must be taken, however, that after flash firing the shell is transferred to a furnace at the temperature of casting and not cooled to room temperature as the shell is very weak after high temperature firing.

In use, the completed shells are preheated to receive molten metal; in the case of aluminum, the preheating temperature is approximately 600 degrees F. The shells of both embodiments may be broken away from the castings by mechanical means because of the low strength characteristics imparted by the plaster of Paris compositions thereof. With respect to the second embodiment, the thin inner and outer layers I0 and 15 are easily broken away, with the intermediate plaster layers offering only limited resistance to the collapse of the shell. Such resistance is further reduced by the weakening of the plaster resulting from its dehydration when exposed to the heat of the casting process.

While in the foregoing we have disclosed embodiments of the present invention in considerable detail for purposes of illustration, it will be understood by those skilled in the art that many of these details may be varied without departing from the spirit and scope of the invention.

We claim:

1. A process for preparing an investment casting shell comprising the steps of applying to a pre-formed pattern a thin coating of a slurry consisting essentially of major proportions of plaster of Paris and a finely-divided refractory material in a non-aqueous liquid binder containing hydrolyzed ethyl silicate, said hydrolyzed ethyl silicate containing an amount of silica within the range of approximately 4 to 15 percent by weight of total nonaqueous binder, at least partially drying said coating and repeating said applying and drying steps until a multilayered shell is formed about said pattern, and thereafter contacting said shell with water to hydrate the plaster of Paris content of the multiple coatings thereof.

2. The method of claim 1 in which the hydrolyzed ethyl silicate of each coating following application thereof is caused to form a silica gel,

3. The method of claim 1 in which there is the further step of heating said shell to dry the same and to destroy the pattern contained therein.

4. In a process for preparing an investment casting shell, the steps comprising applying to a preformed pattern a thin coating of a slurry consisting essentially of major proportions of plaster of Paris and a finely-divided refractory material in a non-aqueous liquid binder containing hydrolyzed ethyl silicate, said hydrolyzed ethyl silicate containing an amount of silica within the range of approximately 4 to l5 percent of total weight of nonaqueous binder, stuccoing the applied coating with a finelydivided refractory material and thereafter at least partially drying said coating, repeating said applying, stuccoing and drying steps until a multi-layered shell is formed about said pattern, and thereafter contacting said shell with water to hydrate the plaster of Paris content of the multiple coatings thereof.

5. The method of claim 4 in which said ethyl silicate contains an amount of silica within the range of approximately 5 to 10 percent of total weight of non-aquous binder.

6. The method of claim 4 in which the hydrolyzed ethyl silicate of each coat following application thereof is caused to form a silica gel.

7. In a process for preparing an investment casting shell, the steps comprising dipping a pre-formed pattern into a stable slurry comprising a major proportion of plaster of Paris intermixed with hydrolyzed ethyl silicate and finely-divided refractory material in a non-aqueous volatile carrier, said hydrolyzed ethyl silicate containing an amount of silica within the range of approximately 4 to percent of total weight of non-aqueous carrier, at least partially drying said coating and repeating said dipping and drying steps until a multi-layered shell is formed about said pattern, and thereafter immersing said shell in water to hydrate the plaster of Paris content of the coatings thereof.

8. The method of claim 7 in which the hydrolyzed ethyl silicate of each coat following application thereof is caused to form a silica gel.

9. The method of claim 7 in which there is the additional step of stuccoing each coat with a finely-divided refractory grain following dipping of said pattern and before drying said coating.

10. In a process for preparing an investment casting shell in which a destructible pattern is successively dipped into a slurry to build up a multiple-layered shell thereon, the steps of forming at least the intermediate layers thereof by successively (l) dipping said pattern into a slurry comprising plaster of Paris as a principal ingredient intermixed with a finely-divided refractory and carried in a non-aqueous volatile binder containing a hydrolyzed silicate, said binder containing an amount of silica within the range of approximately 4 to l5 percent of total weight of binder, (2) at least partially drying said layer, and (3) thereafter repeating said dipping and drying steps.

11. The method of claim 10 in which there is the additional step of stuccoing each layer with a finely-divided refractory grain following said dipping step and before said drying step.

12. The method of claim 10 in which there is the additional steps of contacting all of the layers of said shell with water to hydrate the plaster of Paris content of all of said layers at substantially the same time.

13. The method of claim 10 in which the hydrolyzed silicate of each layer following said dipping step is treated to form a silica gel.

14. A multiple-layered investment casting shell having inner and outer ayers consisting essentially of a mixture of silica and a nely-divided refractory material, and a plurality of intermediate layers composed predominately of hydrated plaster of Paris admixed with a finely-divided refractory material and a minor proportion of a silica binder, said plaster of Paris, refractory material, and silica binder of each layer comprising the residue of a dipcoat containing an amount of silica Within the range of approximately 5 to 15 percent of the weight of the volatile liquid portion of said dipcoat, whereby, upon liring, an investment casting shell is formed having relatively hard ceramic inner and outer layers and a relatively weak plaster-containing core.

References Cited by the Examiner UNITED STATES PATENTS 2,836,867 6/58 Bean 22-193 2,895,838 7/59 Ilenda 106-383 2,948,935 8/60 Carter 22-196 MARCUS U. LYONS, Primary Examiner. 

1. A PROCESS FOR PREPARING AN INVESTMENT CASTING SHELL COMPRISING THE STEPS OF APPLYING TO A PRE-FORMED PATTERN A THIN COATING OF A SLURRY CONSISTING ESSENTIALLY OF MAJOR PROPORTIONS OF PLASTER OF PARIS AND A FNELY-DIVIDED REFRACTORY MATERIAL IN A NON-AQUEOUS LIQUID BINDER CONTAINING HYDROLYZED ETHYL SILICATE, SAID HYDROLYZED ETHYL SILICATE CONTAINING AN AMOUNT OF SILICA WITHIN THE RANGE OF APPROXIMATELY 4 TO 15 PERCENT BY WEIGHT OF TOTAL NONAQUEOUS BINDER, AT LEAST PARTIALLY DRYING SAID COATING AND REPEATING SAID APPLYING AND DRYING STEPS UNTIL A MULTILAYERED SHELL IF FORMED ABOUT SAID PATTERN, AND THEREAFTER CONTACTING SAID SHELL WITH WATER TO HYDRATE THE PLASTER OF PARIS CONTENT OF THE MULTIPLE COATINGS THEREOF.
 14. A MULTIPLE-LAYERED INVESTMENT CASTING SHELL HAVING INNER AND OUTER LAYERS CONSISTING ESSENTIALLY OF A MIXTURE OF SILICA AND A FINELY-DIVIDED REFRACTORY MATERIAL, AND A PLURALITY OF INTERMEDIATE LAYERS COMPOSED PREDOMINATELY OF HYDRATED PLASTER OF PARIS ADMIXED WITH A FINELY-DIVIDED REFRACTORY MATERIAL AND A MONOR PROPORTION OF A SILICA BINDER, SAID PLASTER OF PARIS, REFRACTORY MATERIAL, AND SILICA BINDER OF EACH LAYER COMPRISING THE RESIDUE OF A DIPCOAT CONTAINING AN AMOUNT OF SILICA WITHIN THE RANGE OF APPROXIMATLY 5 TO 15 PERCENT OF THE WEIGHT OF THE VOLATILE LIQUID PORTION OF SAID DIPCOAT, WHEREBY, UPON FIRING, AN INVESTMENT CASTING SHELL F FORMED HAVING RELATIVELY HARD CERAMIC INNER AND OUTER LAYERS AND A RELATIVELY WEAK PLASTER-CONTAINING CORE. 